The invention relates to a method and apparatus for analysing an optical device. The invention relates in particular to a method for the quantitative determination of characteristics of imaging and non-imaging optical systems, and a corresponding device for implementing such a method.
In the manufacture and testing of optical elements and systems, e.g. lenses or imaging optics, it is necessary to evaluate the imaging quality or, more generally, the optical properties. As a rule the properties are described quantitatively for this purpose in the case of higher quality optics. An exact quantitative evaluation of the optical components is also necessary to correct or compensate for inadequacies observed in given optics. Here it is desirable to obtain as wide a coverage of the error types as possible and, in particular, to determine their dependence on wavelength.
A multiplicity of primarily high quality methods are known from the state of the art for determining the properties of optical devices. Such methods include, for example, the star test and other types of schlieren tests, as well as interferometric methods.
Although highly accurate topographical data on the components are determined in the interferometric analysis of the geometry of optical components, no information is obtained on intrinsic properties of the components, e.g. possible material inhomogeneities which influence the quality of the component. In most cases interferometric methods require further optical components whose influence on the measurement must be minimised by correspondingly high quality construction of the components or by compensation. All interferometric methods suffer from the disadvantage that they are technically very demanding and are associated with high costs. Furthermore, these methods are susceptible to interference due to vibrations and air movements, for example, and they are limited to a single wavelength. In addition, interferometric tests on large apertures are technically extremely demanding and correspondingly expensive.
Methods are also known from the state of the art for measuring the optical wave front after it passes through an optical component. In the so-called Hartmann method a diaphragm mask or diaphragm array is inserted in the beam path behind the optical component to be analyzed. The geometric course of the sub-path is determined with a position-resolving sensor, e.g. a CCD camera, from the centroid of the intensity distribution on the sensor and the position of the associated aperture of the diaphragm array, and from this the tilt of the wave front at each aperture of the diaphragm. Unlike interferometric methods, the method referred to is much simpler in structure. However, the Hartmann method suffers from the disadvantage that the achievable spatial resolution is predetermined by the distance between the apertures in the diaphragm mask, and is therefore spatially restricted. The Hartmann method is therefore only of limited application, particularly for measuring small optics. Moreover, non-uniform distribution of the wave front in front of the diaphragm array may, in the Hartmann method, result in incorrect determination of the centroids of the distribution and hence the tilt of the wave front. Suitable optics are therefore required for beam homogenisation and in particular for testing large apertures. Moreover, for a high wave front tilt an appreciable deflection takes place at the diaphragm apertures.
An extension of the Hartmann method, the so-called Shack-Hartmann method, uses a micro-lens array instead of a diaphragm mask. The position-resolving sensor is located in the focal plane of the lens, resulting in higher light sensitivity and considerable insensitivity to inhomogeneous intensity distributions in front of the lens array. Such micro-lens arrays are expensive and costly to manufacture, introduce their own imaging errors into the measurement and are limited in their spatial extension at the top and bottom. In the Hartmann and Shack-Hartmann methods different diaphragm and micro-lens arrays are also required for different geometries of the optics to be analyzed.
Publication DE 103 27 019 A1 discloses a method for determining the imaging quality of an optical imaging system. This method is based on a reconstruction of the wave front, for example by the Gerchberg-Saxton method of prior art. This method, involving a lower technical expenditure, is more accurate than the methods described above. Moreover, the method also enables initially unknown samples to be tested. The disadvantage of this method is that partially coherent light must be used and imaging optics are always required to test the sample. Furthermore, the method is highly susceptible to noise and requires expensive methods for signal processing.